The present invention relates to a method and apparatus for reducing vortex- induced-vibrations ("VIV") and, more particularly, reducing VIV in marine environments by the use of fairings.
Production of oil and gas from offshore fields has created many unique engineering challenges. One of these challenges is dealing with effects of currents on fixed cylindrical marine elements. Such marine elements are employed in a variety of applications, including, e.g., subsea pipelines; drilling, production, import and export risers; tendons for tension leg platforms; legs for traditional fixed and for compliant platforms; other mooring elements for deepwater platforms; and so forth. Ocean currents cause vortexes to shed from the sides of these marine elements, inducing vibrations that can lead to the failure of the marine elements or their supports.
Shrouds, strakes and fairings have been suggested for such applications to reduce vortex induced vibrations. Strakes and shrouds can be made to be effective regardless of the orientation of the current to the marine element. But shrouds and strakes are generally less effective than fairings and generally materially increase the drag acting on the marine element. By contrast, fairings are generally very effective in reducing vibrations due to vortex shedding, and also reduce drag forces on the marine element.
U.S. Pat. Nos. 4,389,487 and 4,474,129 disclose fairings for use with subsea pipes and risers which are provided with means to permit the fairing to rotate around the pipe or riser as would a weathervane in order to maintain an orientation presenting the fairing parallel to the current. However, the subsea environment in which the fairings must operate renders likely the rapid failure of the rotational elements. Further, traditional fairings present a very serious problem should corrosion or marine growth cause the rotational elements to seize up. Such a failure a traditional fairing to rotate would cause excessive drag forces on the marine element should the current shift and no longer align with the "frozen" fairing. As a result, rotatable fairings have, in actual practice, been limited to drilling riser applications in which the risers (together with fairing mounted thereon) are frequently and routinely retrieved and not left in service for extended periods.
An advantage of the present invention is to provide a fairing system that will remain free to weathervane to align with the most effective orientation to the current and which is resistant to fouling from marine growth that could inhibit the rotative freedom necessary to support this weathervaning.